So I typically create a new package called handlers. In that package I create a class file, call it whatever you want, maybe ArmorEventHandler.
Make sure you register the class like it says in the tutorial. It will run the onArmorEvent function whenever the SetArmorModel event occurs. BTW, that event by canceling it, causes the armor to simply not be rendered, hence it appears to be invisible.
I really would recommend you mod in at least 1.7.10, as many of the events that are so helpful, did not exist in 1.6.4. Although that isn't the problem here. I think it used a different syntax for this, try:
The Meaning of Life, the Universe, and Everything.
Alright so I registered it in my preInit but the armor still isn't turning invisible, just to clarify, I'm trying to make it so that I'm wearing a certain armor and that armor will turn me invisible and any other armor worn with it